Ambipolar photoreceptor and method

ABSTRACT

AN AMBIPOLAR PHOTORECEPTOR MEMBER WHICH COMPRISES A SUBSTRATE, A THIN LAYER OF THALLIUM DOPED VITREOUS SELENIUM OR THALLIUM DOPED SELENIUM-ARSENIC CONTAINED ON THE SBSTRATE, AND A LAYER OF VITREOUS SELENIUM OR SELENIUMARSENIC OVERLAYING THE THALLIUM DOPED LAYER. THE PHOTOAND DISCHARGE FOR PHOTOGENERATED CHARGES OF EITHER AND DISCHARGE FOR PHOTOGENERATED CHARGES OF EITHER POLARITY.

Jan. 23, 1973 A. J. ClUFF'lNI 3,712,810

AMBIPOLAR PHOTORECEPTOR AND METHOD Filed Dec. 18, 1970 INVENTOR. ANTHONY J C/UFF/N/ M WZW AT TORNE V United States Patent 3,712,810 AMBIPOLAR PHOTORECEPTOR AND METHOD Anthony J. Ciufiini, Rochester, N.Y., assignor to Xerox Corporation, Stamford, Conn. Filed Dec. 18, 1970, Ser. No. 99,558 Int. Cl. G03g 5/00, 7/00 US. Cl. 96-15 4 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION This application relates to xerography, and more specifically, to a novel photoconductive device and a method of imaging.

In the art of xerography, a xerographic plate containing a photoconductive insulating layer is imaged by first uniformly electrostatically charging its surface. The plate is then exposed to a pattern of activating electromagnetic radiation such as light, which selectively dissipates the charge in the illuminated areas of the photoconductive insulator while leaving behind a latent electrostatic image in the non-illuminated areas. This latent electrostatic image may then be developed to form a visible image by depositing finely-divided electroscopic marking particles on the surface of the photoconductive insulating layer. This concept was originally described by Carlson in US. Pat. 2,297,691, and is further amplified and described by many related patents in the field.

The use of vitreous or amorphous selenium, as described in Bixby in US. Pat. 2,970,906, remains the most widely used photoreceptor in commercial reusable xerography. Vitreous selenium is capable of holding and retaining an electrostatic charge for relatively long periods of time when not exposed to light, and is relatively sensitive to light as compared to most other photoconductive materials. In practice, vitreous selenium for use in xerographic plates is usually given a positive surface charge during the electrical sensitizing operation. This positive charging takes advantage of the better hole conduction through the selenium layer during illumination, in that selenium has a much greater efficient discharge for hole than electron conduction. Therefore, unless specially treated, or other special precautions are taken, selenium generally has a suitable long range for holes and in general must be positively charged.

It should be pointed out that vitreous selenium does conduct both electrons and holes, but that the mobility of holes is approximately ten times greater than that for electrons. Thus, it can be stated that vitreous selenium while possessing a long range for holes has a very short range for electrons. The alfect of this characteristic 011 the Xerographic utility of selenium can best be underice stood by examining the basic process steps of Xerography. As stated above, a vitreous selenium photoconductive layer is first sensitized by placing a uniform electrostatic charge on the surface of the photoconductive insulating material. This uniform charge creates a relatively strong field across the selenium (generally relative to a conductive backing). The vitreous selenium is then exposed to radiation to which it is sensitive, usually in the bluegreen portion of the visible spectrum. The absorption of activating radiation acts to create hole-electron pairs in the selenium at the point of absorption of the impinging radiation. .If the sensitizing charge on the surface of the selenium is negative, positive charge created by the radiation remains at the surface to neutralize existing negative charges while the photogenerated negative charges are repelled by the remaining sensitizing charge to migrate through the selenium toward the conductive backing. When the sensitizing charge on the surface of the selenium is positive, the reverse is true. Electrons created by the radiation remain at the surface to neutralize positive charges and the photogenerated holes or positive charge carriers are repelled to migrate through the selenium to the conductive backing. Inasmuch as selenium has a very short range for electrons, when used with negative charging, the result is that a large number of electrons are trapped in the bulk of the selenium layer, thereby rendering the plate unfit for further use in xerography until the trapped charges are freed. In that selenium has a long range for holes, when used with positive sensitizing charges, trapping is reduced to sufficiently small degree so as not to interfer with the utility of the material for xerographic processes. It therefore has become the usual practice in xerography, when using vitreous selenium, to employ positive polarity sensitizing charges at its surface.

There are, however, some applications in xerography where in a short range for a minority charge carrier constitutes a critical deficiency. For example in xeroradiography, X-rays are used which penetrate the entire film of selenium, generating electron-hole pairs throughout the photoconductive layer, rather than merely near the surface as in the case of normal xerographic imaging. In order to absorb as large a portion of the X-ray energy as possible, xeroradiography requires the use of selenium films considerably thicker than those used in normal xerography. A conventional xerographic plate is one designed for use with visible radiation, and is generally about 20 to microns in thickness. In xeroradiography, however, the selenium films are from about to 200 microns thick. The result of this combination of circumstances is that to be useful in xeroradiography, the photoconductive insulating material must have an appreciable range for both holes and electrons.

Another example where it is desirable that vitreous selenium have a long range for both polarities of charge carriers is in obtaining a reversal of the image to be reproduced in the normal xerographic process. In this case, if the normal xerographic plate is charged negatively and then the steps of the xerographic process carried through, including development with carriers and toners as described for the normal xerographic process, there is obtained a negative or reversal image of the copy being reproduced. Thus, if the plate has a long range for both 3 polarities of charge carriers it is possible merely by altering the polarity of the sensitizing charges to obtain either a positive or reversal reproduction of the subject matter being reproduced.

US. Pat. $077,386 to Blakney et al. describes one technique for treating selenium whereby the material acquires the property of having a long range for both polarities of charge carriers. This technique involves doping the selenium with a small amount of metal such as chromium, nickel, zinc, iron, calcium, titanium, or other similar material.

There has now been discovered, an alternative method of rendering selenium and selenium-arsenic alloys suitable for use in methods which require a satisfactory carrier range for both polarities of charge carriers.

OBJECTS OF THE INVENTION It is, therefore, an object of this invention to provide a novel device, containing a selenium or selenium alloy photoconductive layer which exhibits ambipolar characteristics.

It is another object of this invention to provide a novel photosensitive device which exhibits ambipolar characteristics.

It is a further object of this invention to provide a system utilizing a photoconductive layer which exhibits a satisfactory range for charges of both positive and negative polarity.

SUMMARY OF THE INVENTION The foregoing objects and others are accomplished in accordance with this invention by providing a xerographic member which provides good charge acceptance and discharge for charges of both polarities, and which may be referred to as an ambipolar photoreceptor. The instant concept is based upon the use of a photoreceptor which utilizes a hole trapping layer interposed between the substrate material and the main photoreceptor layer. One embodiment of this invention comprises a conducting substrate overlayed with a thin layer comprising thallium doped selenium or a selenium-arsenic alloy, and a relatively thick bulk layer of vitreous selenium or a selenium alloy overlaying the thallium doped layer.

BRIEF DESCRIPTION OF THE DRAWING The advantages of the instant invention will become apparent upon consideration of the following disclosure f the invention, especially when taken in conjunction with the accompanying drawing, wherein:

The figure represents a schematic illustration of one embodiment of an improved photoreceptor device of the for use in the instant invention.

DETAILED DESCRIPTION OF THE DRAWING In the drawing, reference character 1-0 illustrates one embodiment of an improved photoreceptor device of the instant invention. Reference character 11 designates support member which is preferably an electrically conductive material. The support may comprise a conventional metal such as brass, aluminum, steel, or the like. The support may also be of any convenient thickness, rigid or flexible, and in any suitable form such as a sheet, web, cylinder, or the like. The support may also comprise other material such as metallized paper, plastic sheets covered with a thin coating of aluminum or copper iodide, or glass coated with a thin conductive layer of chromium or tin oxide. An important consideration is that the support member be somewhat electrically conductive, or have a conductive surface or coating, and that it be str ng enough to withstand a certain amount of handling. Reference character 12 designates a thin interlayer which may comprise vitreous selenium or a selenium-arsenic alloy containing a small, but critical amount of thallium in the range of about to 500 parts per" million by weight. Reference character 13 represents a photoconductive layer which may comprise vitreous selenium or a seleniumarsenic alloy overlaying layer 12. When alloying the selenium with arsenic, the arsenic may vary from about 0.1 to 2.5 percent by weight.

The thickness of layer 12 should be from about 0.5 to 15 microns, while the thickness of the overlaying selenium or selenium-arsenic layer is not particularly critical and may range from about 10 to 200 microns. In general, thicknesses in the range of about 20 to microns are particularly satisfactory for the top layer when used for conventional xerography.

The two photoreceptor layers of the instant invention may be prepared by any suitable technique. A preferred technique includes vacuum evaporation where each photoconductive layer is sequentially evaporated onto its corresponding base material. In this technique, the first layer, which may comprise selenium doped with a small amount of thallium, or a second or top layer which may comprise undoped selenium, may be each evaporated by separate steps under vacuum conditions varying from about 10- to 10" torr. In another embodiment of this particular technique, the two photoreceptor layers may be continuously vacuum evaporated, one after the other, in the same vacuum chamber without breaking the vacuum, by sequentially activating the two separate sources of thallium doped selenium and selenium.

Another typical technique which may be used includes co-evaporation, wherein the approriate amount of each material for the alloy layers is placed in separate heated crucibles maintained under vacuum conditions with a source temperature of each alloy constituent being'controlled so as to yield the appropriate percentage of the alloy desired. This technique is more fully described in copending patent application, Ser. No. 566,593, filed on July 20, 1966, and now US. Pat. 3,490,903.

Another typical method includes flash evaporation under vacuum conditions similar to those defined in co-evaporation, in which a powdered alloy such as the thallium doped selenium is selectively dropped into a heated crucible maintained at a temperature of about 300 to 600 C. The vapors formed by the heated mixture are evaporated upward onto a substrate supported above the crucible.

In all of the above methods, the substrate onto which the photoconductive material is evaporated is maintained at a temperature of about 50 to 80 C. If desired, a water cooled platen or other suitable cooling means may be used in orderto maintain a constant substrate temperature. In general, a selenium layer thickness of about 60 microns is obtained when vacuum evaporation is continued for about 1 hour at a vacuum of about 10- torr. at a crucible temperature of about 280 C. US. Pat. 2,803,542 to Ullrich; 2,822,300 to Mayer et al.; 2,901,348 to Dessauer et al.; 2,963,376 to Schatrert; and 2,970,906 to Bixby all illustrate vacuum evaporation techniqes which are suitable in the formation of alloy layers of the instant invention. The crucibles which are used for the evaporation of photoreceptor layers may be of any inert material such as quartz, molybdenum, stainless steel coated with a layer of silicon monoxide, or any other suitable equivalent material. In general, the selenium or selenium alloy being evaporated is maintained at a temperature of above its melting point.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The following examples further specifically define the present invention with respect to a method of making and imaging a composite photoreceptor suitable for use with either positive or negative charging.

EXAMPLE I A plate made according to the instant invention is prepared as follows. Two stainless steel evaporation boats having a surface coating of silicon monoxide are placed in a vacuum evaporation chamber. The first boat contains a source of xerographic grade selenium alloyed with 0.5

weight percent of elemental arsenic. The selenium is xerographic grade having a purity of about 99.999 percent and available from Canadian Copper Refiners. The alloy is in the form of pellets ranging from about M; to 71 inch in diameter. The second boat contains the same seleniumarsenic alloy, which is additionally doped with thallium in a concentration of about 10 parts per million by weight of the alloy composition. Each boat is connected directly to a source of electrical power adaptable to control the temperature of the respective boat. A brass substrate having a chromated interface is suspended about six inches above the evaporation boats. The thallium doped selenium-arsenic alloy boat is then heated to a temperature of about 300 C. for about 15 minutes to form a 10-micron layer of the thallium doped arsenic-selenium on the brass substrate. At the end of 15 minutes, the power of this boat is turned ofl and the boat covered with a metal shutter. The boat containing the undoped arsenic-selenium alloy is then heated to a temperature of about 350 C. for 50 minutes to form a SO-micron layer of arsenic-selenium over the thallium doped arsenic-selenium layer. At the end of this time, the vacuum chamber is cooled to room temperature, the vacuum broken, and the composite photoconductive plate removed from the chamber.

EXAMPLE II A control plate consisting of a single 60-micron layer of 99.5 percent selenium and 0.5 percent weight arsenic is prepared on a brass substrate having a chormated interface by the method of Example I. The boat is heated to about 350 C. for about 1 hour to form a 60-micron alloy layer. Both of the above plates are then tested to measure their charge acceptance for both negative and positive polarity. The plate made according to Example I, which illustrates one embodiment of the instant invention, accepts 1000 volts of positive charge and is suitable for xerographic imaging in the conventional xerographic manner. This plate also accepts 800 volts of negative charge and also is capable of being used in the conventional xerographic manner using negative charging. The plate prepared by Example II, which does not utilize the thallium doped layer, accepts the same 1000 positive volts on positive charging, as would be expected. However, upon negative charging, this plate, which did not contain the thallium doped layer, accepts only 90 volts of negative charging which is almost a factor of 10 less charge acceptance for negative charging than the plate of Example I.

EXAMPLE III An additional plate containing a SO-micron layer of a selenium-arsenic alloy containing 99.5 weight percent selenium and 0.5 weight percent arsenic over a 10 micron layer of a thallium doped selenium-arsenic alloy (99.5 weight percent Se-0.5 weight percent As) is formed by the method of Example I. In this embodiment, the thallium doped layer contains 100 parts per million thallium and the substrate comprises an aluminum oxide coated drum blank. The size of the drum is about three inches in diameter and ten inches long and is suspended six inches above the evaporation boats.

EXAMPLE IV A plate is made according to the method of Example II and comprises a 60-Inicron coating of 99.5 weight percent selenium-0.5 weight percent arsenic. The alloy is coated onto an aluminum drum blank containing an aluminum oxide interface. The diameter of the drum is about three inches and ten inches long. The plates of Examples III and IV are then tested for their charge acceptance for both positive and negative polarity as in Examples I and II. The plate made according to the instant invention (Example III) exhibits 1000 volts charge acceptance at positive charging and a 1500-volt charge acceptance for negative charging. On the other hand, the control drum, which does not have the thallium doped interlayer, also exhibits the same 1000 volts charge acceptance for positive charging, but only 600 volts charge acceptance for negative charging. Both of these plates are suitable for conventional xerographic imaging which includes charging, exposure, and development of the latent electrostatic image.

EXAMPLE V An additional plate comprising a BO-micron coating of 99.5 weight percent selenium-0.5 weightpercent arsenic coated on an aluminum drum blank containing an aluminum oxide interface is prepared by the method of Example III. The thallium doped layer is 10 microns thick, with the thallium concentration being about parts per million by weight.

The drum is first tested by charging the surface of the plate to an acceptance potential of 800 volts negative potential. The drum is then discharged by exposure to a pattern of visible light. Following exposure, a contrast potential of about 560 volts remains onthe drum surface in the unexposed areas. All of the surface charge is then erased from the surface of the drum by uniform exposure to a cool white erase lamp. The above cycle is repeated 8 additional times. At the end of 9 cycles, no residual build-up is observed with the contrast potential remaining relatively constant.

The above charging, exposure and erase cycle is then carried out by using a negative acceptance potential of 1000 volts. Upon exposure, a contrast potential of 700 volts remains on the drum surface in the unexposed areas. The surface charge in the unexposed areas is then erased by exposure to the cool white erase lamp. The above cycle is repeated 10 additional times. At the end of 11 cycles, no residual build-up is observed, with the contrast potential remaining relatively constant.

Although specific components and proportions have been stated in the above description of the preferred embodiments of this invention, other suitable materials and procedures such as those listed above, may be used with similar results. In addition, other materials and changes may be utilized which synergize, enhance, or otherwise modify the photoreceptor and method of use.

Other modifications and ramifications of the present invention would appear to those skilled in the art upon reading the disclosure. These are also intended to be within the scope of this invention.

What is claimed is:

1. An ambipolar photoreceptor member which comprises an electrically conductive substrate, a thin layer of thallium doped vitreous selenium about 0.5 to 15 microns thick contained on said substrate, said thallium being present in a concentration of about 5 to 500 parts per million by weight, and a layer of undoped vitreous selenium overlaying the thallium doped layer.

2. The member of claim 1 in which the thin layer comprises a thallium doped selenium-arsenic alloy and the undoped layer comprises a selenium-arsenic alloy, in which the concentration of arsenic in both layers is about 0.1 to 2.5 weight percent, with the balance substantially selenium.

3. A method of imaging which comprises:

(a) providing an ambipolar photoreceptor member which comprises an electrically conductive substrate, a thin layer of thallium doped vitreous selenium or a thallium doped selenium-arsenic alloy about 0.5 to 15 microns thick contained on said substrate, said thallium being present in a concentration of about 5 to 500 parts per million by weight, and a thin layer of a vitreous selenium or a selenium-arsenic alloy overlaying the thallium doped layer, with the concentration of the arsenic in the selenium-arsenic alloy being about 0.1 to 2.5 weight percent, with the balance substantially selenium;

(b) uniformly electrostatically charging the member to a negative potential; and

7 8 (c) exposing said charged member to a source of acti- 3,501,343 3/1970 Rezensburger 961i5 X vating electromagnetic radiation to form a latent elec- 3,312,548 4/ 1967 Straughan 96--1.5 trostatic image. 3,170,790 2/1965 Clark 96-15 X 4. The method of claim 3 in which the latent image is developed to form a vlslble lmage- 5 GEORGE F. LESMES, Primary Examiner References Cited J. R. MILLER, Assistant Examiner UNITED STATES PATENTS 3,508,918 4/1970 Le-vy 96-l.5

3,427,157 2/1969 Cerlon 96--1.5 X 961 R; 117-21s; 250-65 25252-501 

